Goorissen



March 10, 1964 .1. GOORISSEN 3,124,636

METHOD OF AND APPARATUS FOR HEAT TREATMENT OF SEMI-CONDUCTOR MATERIAL Filed Sept. 12, 1960 INVENTOR 7- MM AGENT United States Patent 3,124,686 METHQD OF AND APPARATUS FOR HEAT TREAT- MENT 0F SEMI-CONDUCTOR MATERIAL Jan Goorissen, Eindhoven, Netherlands, assignor to North American Phiiips Company, Inc, New York, N.Y., a

corporation of Delaware Filed Sept. 12, 196i), Ser. No. 55,372 Claims priority, application Netherlands Sept. 24, 1959 4 (Ilaims. (Cl. 25083.3)

In the production of semi-conductor material and the manufacture of bodies from these materials use is frequently made in practice of heat treatments, for example for separating the material from the gas phase, for purifying it or working it up into single-crystal bodies. Examples of these treatments are treatments in which a body of semi-conductor material in the shape of a rod grows by solidification of a melt of the material, such as zonemelting treatments, which may either be performed in an elongated crucible according to Pfann or without the use of a crucible according to Keck, and the drawing of crystals from a melt of semi-conductor material according to the method of Czochralski. A further example is the deposition of semi-conductor material on a support, for example by sublimation or by decomposition of volatile compounds when heated.

In such treatments generally changes occur, for example melt formation, solidification and changes in shape, which are perceptible with the eye so that the course of the treatment can be followed and, if required, controlled. In treatments in which a body of semi-conductor material grows to form a rod by solidification of a melt it is of importance that the thickness of the rod during its growth is supervised and controlled by the observer, for example by controlling the supply of heat to the melt. When semi-conductor material is segregated from the gas phase on to a support it is desirable that the degree of growth of the deposited material should be followed by observation and controlled according to this observation in order to ensure the most eflicient and uniform deposition.

Since with semi-conductor materials it is undesirable that heat-treatments should be performed in air, such a treatment is carried out in an enclosure in which the composition of the atmosphere can be controlled. Generally the enclosure has a wall made at least in part of a transparent material such as vitreous quartz in order to permit observation.

It has, however, been found that frequently a coating of the semi-conductor material is deposited on the wall of the enclosure and prevents direct observation of the material to be treated. Such a deposit is formed in particular when elevated temperatures are used and/ or at a very low gas pressure in the enclosure, for example if the heat treatment is carried out in a vacuum.

It is an object of the invention to permit continuous observation of the phenomena occurring during heat treatment of semi-conductor materials. The invention utilizes the property of a semi-conductor material that it transmits infra-red radiation over a certain wavelength range in contradistinction to, for example, a metal, so that infra-red radiation of a wavelength within this range is also transmitted by the deposit of the semi-conductor material on the wall. In general a conventional transparent wall portion of vitreous quartz is also permeable to such radiation.

According to the invention during the heat treatment an infra-red image converter is used for observing the material under treatment. The term infra-red image converter as used herein denotes in general a device capable of converting an infra-red radiation image into an ice image of visible light. The image converter may be an infra-red viewer known in the art, as described in Proceedings of the IRE. 47 (1959), No. 5, page 904, or a flat image intensifier sensitive to infrared radiation. A camera tube sensitive to infra-red radiation may be used according to principles known from television, the signal delivered by this tube being converted into a visible image in a display tube. The infra-red image converter must be sensitive to the infra-red radiation transmitted by the semi-conductor material concerned. The wavelength range of the transmitted radiation is bounded at the short wave end by an absorption boundary of the semi-conductor material which is related to the energy spacing, hereinafter referred to as the band spacing, between the valency band and the conduction band of the semi-conductor material and which is situated at a smaller wavelength according as the band spacing is larger. In general the heated semi-conductor material itself may serve as the source of infra-red radiation, however, this material may alternatively be irradiated by infra-red radiation.

It has also been found that the use of an infra-red image converter may have further advantages, for example when mists occur during the treatment which disperseinfra-red radiation in a materially smaller degree and, if they consist of semi-conductor material, absorb this radiation to a lesser extent than visible radiation. Furthermore, when a layer of semi-conductor material is deposited on a support, the thickness of the layer can be determined during the deposition by means of interference of reflected infra-red radiation and this generally is not possible with visible radiation due to the frequently high absorptive power of the semi-conductor material for visible light.

Although observation by means of an infra-red image converter generally will be satisfactory when using a conventional wall portion of vitreous quartz, use may also be made of wall portions made from other material transmitting infra-red radiation, for example portions consisting of infra-red transmitting glasses or a window made of the same material or of another semi-conductor material having a band spacing equal to or greater than that of the material to be treated.

The invention will now be described more fully with reference to the accompanying drawing in which the figure is a vertical sectional and partly diagrammatical View of an apparatus for zone melting without the use of a crucible.

In this figure, reference numeral 1 denotes a vertical tube of vitreous quartz in which a vertical rod 2 of silicon is arranged. This rod comprises two solid portions 3 and 4 secured at their upper and lower ends respectively in holders (not shown) and an intermediate substantially drop-shaped molten zone 5 heated by means of a high-frequency coil 6. Due to the high surface tension of the melt the molten zone 5 is held in place between the two rod portions 3 and 4-. The vitreous quartz tube 1 and the rod 2 are gradually lowered vertically by means (not shown) in the direction indicated by an arrow, whereas the coil 6 does not change position so that the molten zone 5 passes through the silicon rod 2, the rod portion 3 gradually melting on at its lower end and the rod portion 4 gradually growing at its upper end.

The shape and the length of the molten zone 5 and the diameter or" the growing rod portion 4 can be controlled by adjusting the strength of the current in the high-frequency coil, which current is provided by a highfrequency generator (not shown). Owing to the high temperature of the melt, part of the silicon will evaporate from the zone 5 and be partly deposited as an opaque coating 7 on the inside of the vitreous quartz tube 1 so that observation of the zone with the naked eye is impeded. The apparatus also includes an infra-red image converter shown diagrammatically in the drawing as an infra-red viewer 3 comprising a concentrating lens or lens system for infra-red radiation 9 and a vacuum tube ltl consisting of a photo-cathode 11 which is sensitive to infra-red radiation of a wavelength exceeding 1.1 the absorption boundary of silicon, a set of electrodes (not shown) and a cathodoluminescent screen 12. During the treatment the heated molten zone 5 emits infrared radiation of which the part having wavelengths exceeding 1.1 passes substantially unimpeded through the silicon layer 7 deposited on the inner wall of the vitreous-quartz tube l and can be collected by the infrared viewer 8. An infra-red image of the molten zone 5 is projected onto the photo-cathode 11 by means of the lens or lens system 9. The electrons emitted by the photo-cathode 11 are projected with the aid of the electrodes (not shown) provided in the tube onto the screen 12 so that a visible image of the molten zone 5 is produced on this screen. By this image the observer 13 is enabled to follow the zone-melting process without the use of a crucible and, if required, to control this process, for example by varying the high-frequency current supplied to the coil by the generator, without being impeded by the silicon deposit 7 on the vitreous quartz tube 1. l

If the rod portions 3 and 4 are irradiated by infrared radiation, they can also be observed by means of the image converter. It has been found that generally normal daylight contains a sufiicient amount of infra-red radiation for this purpose.

Although in this example the process of zone-melting silicon without the use of a crucible has only been described, it goes without saying that an infra-red image converter can also be used in heat treatments of other semi-conductor materials and in further heat treatments, for example zone melting in a crucible, drawing crystals from the melt, deposition of semi-conductor material by sublimation or decomposition of volatile compounds, etc. without departing from the scope of the present invention.

What is claimed is:

1. A method of treating semiconductor material within an enclosure containing a portion substantially transparent to infra-red radiation and controlling the said treatment, comprising the steps of heating the semiconductor material as part of the treatment under conditions at which semiconductor vapors are formed that condense on the substantially transparent enclosure portion forming a layer that is substantially opaque to visible radiation but substantially transparent to infra-red radiation, and utilizing an infra-red image converter located outside the enclosure for detecting through the said substantially transparent enclosure portion and layer an infra-red image of the heated semiconductor material and for converting that infra-red image into a visible image of the heated semiconductor material which can be observed for the purpose of controlling the treatment of the material.

2 A method of zone-melting silicon semiconductor material within an enclosure containing a low-pressure protective atmosphere and including a wall portion substantially transparent to infra-red radiation and controlling the said treatment, comprising the steps of heating and melting the semiconductor material as part of the treatrnent whereby vapors of the material are formed within the enclosure that deposit on the said wall portion forming a layer that is substantially opaque to visible radiation but substantially transparent to infra-red radiation, and utilizing an infra-red image converter located outside the enclosure for detecting through the said substantially transparent wallportion and layer an infra-red image of the heated semiconductor material and for converting that infra-red image into a visible image of the heated semiconductor material which can be observed for the purpose of controlling the treatment of the material.

3. Apparatus for heat treatment of a semiconductor material and for controlling that treatment, comprising an enclosure in which the semiconductor material is located, said enclosure containing a portion substantially transparent to infra-red radiation, means for controlling the atmosphere within the enclosure, means for heating the semiconductor material while in the enclosure under conditions at which an infra-red image of the heated semiconductor material is generated and vapors are formed that condense on the substantially-transparent enclosure portion forming a layer that is substantially opaque to visible radiation but substantially transparent to infra-red radiation, and means located outside the enclosure but adjacent the said substantially transparent portion for detecting the said infra-red image therethrough and for converting the said infra-red image into a visible imagewhich can be observed for the purpose of controlling the treatment of the material.

4. Apparatus for zone-melting of silicon semiconductor material and for controlling it, comprising an enclosure in which the semiconductor material is located, said enclosure being of a material substantially transparent to infra-red radiation, means for providing a low pressure protective atmosphere withinv the enclosure, means for heating the semiconductor material while in the enclosure to a temperature at which the material melts and vapors of the material are formed which deposit on the enclosure walls forming a layer opaque to visible radiation but transparent to infra-red radiation and under the conditions at which an infra-red image of the heated semiconductor material is generated, and means located outside the enclosure but adjacent thereto for detecting the said infra-red image therethrough and for converting the said infra-red image into a visible image which can be observed for the purpose of controlling the zonemelting of the material.

References Cited in the file of this patent UNITED STATES PATENTS Loy Apr. 24, 1962 OTHER REFERENCES Infrared Oil-Film Camera Shows Heat Patterns, Electronics, April'195'6, page 190. 

1. A METHOD OF TREATING SEMICONDUCTOR MATERIAL WITHIN AN ENCLOSURE CONTAINING A PORTION SUBSTANTIALLY TRANSPARENT TO INFRA-RED RADIATION AND CONTROLLING THE SAID TREATMENT, COMPRISING THE STEPS OF HEATING THE SEMICONDUCTOR MATERIAL AS PART OF THE TREATMENT UNDER CONDITIONS AT WHICH SEMICONDUCTOR VAPORS ARE FORMED THAT CONDENSE ON THE SUBSTANTIALLY TRANSPARENT ENCLOSURE PORTION FORMING LAYER THAT IS SUBSTANTIALLY OPAQUE TO VISIBLE RADIATION BUT SUBSTANTIALLY TRANSPARENT TO INFRA-RED RADIATION, AND UTILIZING AN INFRA-RED IMAGE CONVERTER LOCATED OUTSIDE THE ENCLOSURE FOR DETECTING THROUGH THE SAID SUBSTANTIALLY TRANSPARENT ENCLOSURE PORTION AND LAYER AN INFRA-RED IMAGE OF THE HEATED SEMICONDUCTOR MATERIAL AND FOR CONVERTING THAT INFRA-RED IMAGE INTO A VISIBLE IMAGE OF THE HEATED SEMICONDUCTOR MATERIAL WHICH CAN BE OBSERVED FOR THE PURPOSE OF CONTROLLING THE TREATMENT OF THE MATERIAL. 